Particle preaccelerator arrangement

ABSTRACT

The invention relates to an arrangement for accelerating, to a predetermined velocity at least one bunch from a bunched beam of particles, the length of said bunch corresponding to one cycle of the high frequency of the accelerator. In this arrangement the injection voltage V of the beam is raised to a value larger than the cut-off voltage Vc of the accelerator, exclusively during a short period. Said injection voltage V is created by the addition of a pulsed voltage Vo and high-frequency voltages V1 and V2 generated in resonant cavities 3 and 4 disposed in the trajectory of the beam and tuned respectively to a sub-multiple of said high frequency.

United States Patent 1 Leboutet et al.

[ Oct. 30, 1973 PARTICLE PREACCELERATOR ARRANGEMENT Inventors: Hubert P.Leboutet; Andre M.

Bensussan, both of Paris, France Foreign Application Priority Data Apr.28, 1970 France 70l5527 us. c1. ..328 233, 3 13/63, 3 1 515. 1 1

Int. Cl. H0lj 23/20 Field of Search ..3l3/63; 328/233; M .3.1.1lll..-.4.21 ,243.; 3.3. .17

References Cited UNITED STATES PATENTS Webster 328/233 UX Takeda et a1.328/233 3,454,818 7/1969 Softer et al. 3l5/5.4l 3,275,867 9/1966Tsuchimoto 313/63 2,974,252 3/1961 Quate 3l5/5.43 X

Primary Examiner-Alfred L. Brody Attorney-Cushman, Darby & Cushman [57]ABSTRACT The invention relates to an arrangement for accelerating, to apredetermined velocity at least one bunch from a bunched beam ofparticles, the length of said bunch corresponding to one cycle of thehigh frequency of the accelerator.

In this arrangement the injection voltage V of the beam is raised to avalue larger than the cut-off voltage V of the accelerator, exclusivelyduring a short period. Said injection voltage V is created by theaddition of a pulsed voltage V and high-frequency voltages V and Vgenerated in resonant cavities 3 and 4 disposed in the trajectory of thebeam and tuned respectively to a sub-multiple of said high frequency.

8 Claims, 4 Drawing lFigures PARTICLE ACCELERATOR VOLTAGE, SOURCE\you/AGE GENERATOR J, 30 MASTER j= oscmmon 7 AMPLIFIER PHASE SHFTFREQUENCY FREQUENCY 1 MULTIPUER AMPUHER MULHPUER "\SYNCHRQNIZER PARTICLEPREACCELERATOR ARRANGEMENT The present invention relates to linearparticle accelerators. In these kind of equipment, a particle beampasses through a linear structure formed by a linear arrangement ofresonant cavities containing openings for the passage of the beam andsupplied from a highfrequency power source. It is well known that theseparticles are, during their displacement, progressively regrouped intobunches which succeed one another at the output of the accelerator, atthe frequency of said power source.

However, in certain applications, it may be desirable to accelerate onlyone bunch or package of particles, for example in order to simulate asingle nuclear event or to study the very short lifetime of metastablechemical elements. This requirement means that the length of the packagecorresponds at the most to one cycle of the high acceleration frequency.Several solutions have already been proposed for this problem.

Thus the operation of a linear particle accelerator being generally of apulsed nature, the pulse duration may be reduced down to a valuecorresponding to only one cycle of the high acceleration frequencyhowever, this becomes hardly feasible as soon as the acceleratingfrequency exceeds 100 Mc/s, value which is a rather low value indeed.

Another solution consists in subjecting the beam to transversedeflection, either electrical or magnetic, so that it can only enter thelinear accelerator structure during the brief instants when there is nodeflection.

However, this may substantially affect the geometric and energeticcharacteristics of the beam.

It is an object of this invention to avoid such drawbacks to thiseffect, the particles outside the useful bunch being completelyeliminated.

On the other hand, in the device of the invention, the acceleratedparticle bunch is that at the peak of the HF wave which, at thisinstant, has a zero transverse component therefore there is no undesireddefocusing of the beam.

According to the invention, there is provided a buncher andpreaccelerator arrangement for preaccelerating to a predeterminedvelocity, particle bunches intended to be injected into the acceleratingsections of a particle accelerator, said arrangement comprising aparticle source means for extracting said particle from said source afirst cavity resonator and at least one second cavity resonatorpositioned along the path of said particles, cavity resonators beingprovided with respective openings for the propagation of said particlestherethrough means for applying an electromagnetic energy at the highfrequency f to said first cavity resonator for bunching said particlestherein and means for applying an electromagnetic energy at the highfrequency f, to said second cavity resonator for preaccelerating onebunch at a time at predetermined time intervals said high-frequencies fand f being sub-multiples of the operating frequency f,, of saidaccelerator, and said frequencies f, and f being harmonically related.

In a better understanding of the invention and show how the same may becarried into effect, reference will be made to the following descriptionand the appended drawings among which FIG. 1 illustrates an arrangementaccording to the present invention FIGS. 2 to 4 are explanatory diagramsof this arrangement.

The arrangement shown in FIG. 1 comprises a sound 1 emitting an electronbeam 2, a first resonant cavity 3 and a second cavity 4. They are:traversed at 5 and 6 by the beam 2 which enters the first section of alinear accelerator of any known type which, therefore, has not beenillustrated in detail. The positive pole 9 of a pulsed voltage source 7is connected to the cavities 3 and 4, whereas the negative pole 8thereof is connected to the cathode.

The accelerator is operating at thehigh frequency and the high frequencypower is issued from a klystron tube 13 through the line 14.

This klystron operating at the frequency f is supplied from a pulsedvoltage generator 30 having a recurrence frequency equal to 7. Theresonant cavities 3 and 4 are respectively supplied with high frequencysignals of frequencyf, F,,/l0 andf =f,/2 through the lines 11 and 12,the reason for this choice being explained hereinafter. The masteroscillator is a high stability generator 15 producing a low-powerhigh-frequency output of frequency f One part of this low-powerhigh-frequency energy which is amplified by an amplifier 16, is appliedto the cavity 3 after conventional level and phase adjustment in avariable attenuator 17 and a phase shifter 27 respectively, whereas theother part of this energy is ap' plied to a frequency multiplier 18which multiplies the frequency by 5.

One part of the energy issued from said multiplier 18, and amplified inthe amplifier 19, is applied to the cavity 4, the other part of theenergy being injected into a multiplier 21 which multiplies itsfrequency by 2. The energy issued from said multiplier 21 is thenapplied to amplifier klystron 13, through a phase shifter 23 and avariable attenuator 22.

The instant at which the voltage pulse produced by the pulse generator 7is triggered, is determined by the application to said generator 7 of asynchronising signal, adjusted by a phase-shifter 25 and produced by thesynchroniser 26 in accordance with the phase of the high-frequencysignal coming from the master oscillator l5.

Thus, the cavity 3, the cavity 4 and the klystron 13 are continuouslysupplied with high-frequency power, of frequency respectively equal toF,,/ 10 f,,/2 and f,,. The klystron l3 fed by the voltage generator 30at an arbitrary recurrence frequency F is operating at highfrequency f,equal to the high-frequency of the particle accelerator l0 and suppliesit.

The operation of this kind of system, which is described in order tosimplify matters in relation to the case in which it is desired toaccelerate a single bunch of electrons through the accelerator, is asfollows The pulse generator 7 produces a single voltage pulse ofamplitude V and duration 10 T this being equal to one cycle of thefrequency fl,/l0 produced by the master generator 15. This voltagedetermines the emission, by the cathode of electrons which are injectedinto the cavity 3 at a velocity corresponding to the voltage V Into thefirst cavity 3, the electrons experience an increase in velocitycorresponding to the high frequency voltage V, which prevails in thedrift space 5 of cavity 3. These electrons are then bunched in bunches.

In the second cavity 4, one of these bunches experiences a freshincrease in velocity corresponding to the high-frequency voltage V whichprevails in the drift space 6 of said cavity. The final velocity of saidbunch of electrons is determined by the sum V of the three voltages V,V, V However, the theory of operation of a particle'accelerator showsthat there is a value 'of the injection voltage below which the velocityof the particles is too low for any of them to be captured by theeffective accelerating wave in the accelerator l0, and above which thecurrent supplied by accelerated particles increases rapidly with theinjection voltage. This particular voltage will, throughout thefollowing, be called the. cut-off voltage V I FIG. 2 illustrates thisfact by means a diagram on which the accelerated current [correspondingto a typical accelerator has been plotted versus the injection voltage Vcomprised between and V As shown, the

. accelerated current 1 increases rapidly.

the accelerating wave, i.e., the acceleration thereof.

However, to obtain the electron package having a length corresponding toone cycleat the high frequency, the device shown in FIG. 1 requiresspecial control. This will be shown with particular reference to FIG. 3.I

FIG. 3 illustrates the variations (plotted on the ordinates) inthevoltages V,,, V,, V, referred to hereinbefore as a function of time.

The voltage V, is adjusted to a value much lower than the cut-offvoltage V,- it is added toit the alternating voltage V, of frequencyf,,/l 0 which is set to a value such that the sum V, V, is equal to VThe alternating voltage V' of frequency f /2 is alone to cause thevoltage V; to be exceeded, and consequently brings bunch.

This hunch is very narrow.lts length corresponds to that of an' are 40of the sinusoidal signal of frequency f,,/2 as shown in FIG.'3,thelength of this'arc being consequently equal to the length of a completeoscillation 42 at the high frequency f,,.

multaneously, and this is why the phase-adjusting devices described inFIG. 1 are needed. It should be noted, too, that it is necessary, inorder to obtain a single electron bunch, that only one are 40 of thesinusoidal voltage V, of frequency f,,/2 should exceed the value V V, V,which requires, in practice, that the frequencies of the high-frequencysignals applied respectively' to cavities and klystron tube should bechosen as indicated hereinbefore, and in particular, explains the choiceof an odd number for the ratio between the frequencies of the twocavities.

aboutthe acceleration of an electron However,- other values of thesefactors can be reover a period equal to 10 T,,, corresponding to thefrequency f /IO a pulse of this kind can be produced in a known manner.

However, the length of the accelerated electron bunch extends over onlya period T corresponding to the high frequency f, but a high-currentpulse of this kind cannot be obtained in the priort art devices, asalready stated.

By way of example, it may be indicated that for an accelerating highfrequency wave of l300Mc/s, the voltages V, 50 kV, V, 30 kV, V 40 kV canbe used. The value of the voltage V applied to the cathode, is a muchlower value than the KV required in the case of conventional electronguns.

This advantage in terms of voltage values, may be accompanied by afurther advantage which is concerned with the characteristics of theaccelerated pulse.

Thus, either by slightly detuning the cavities or changing slightlytheir relative phase, it is possible to modify the time dependance lawof the velocity modulation applied to particles, so that, aftertraversing an appropriate length of field" free drift space, thebunching of the particles and therefore the density of the bunchesformed are improved. If, subsequently the beam is passing through aconventional premodulating cavity, which is either a separate one or'isintegral part of the accelerating structure, said cavity being fed atthe nominal frequency of the latter, this conventional premodulatingcavity gives a time-compression effect, in the current pulse produced bythe electron gun, arrangement without any particle loss and consequentlythere results an improvement in the peak value of current delivered bythe accelerator.

The embodiment of the invention as described hereample.

In particular, themeans used to create the highfrequency signals appliedto the different elements of y the device can differ from'thosedescribed, the order of the resonant cavities can be reversed and theirnumber be in excess of two the pulse-operated electron gun, which is ofthe diode type inthe description, may be of the triode type, supply by adirect'voltage applied between cathode and anode, beam injection beingcontrolled by a grid finally, the accelerated particles can be eitherelectrons or ions. I I

What is claimed is I. A buncher and preaccelerator arrangement forpreaccelerating, to a predetermined velocity, particle bunches intendedto be injected into the accelerating sections of a particle accelerator,said arrangement comprising a particle source means for extracting saidparticles from said source a first cavity resonator and at least onesecond cavity resonator positioned along the path'of said particles,said cavity resonators being provided with respective openings for thepropagation'of said particles through said cavity resonators means forapplying ,an electromagnetic energy at the high-frequency f, to saidfirst cavity resonator for bunching said particles therein and means forapplying an electromagnetic energy at the high-frequency f to saidsecond cavity resonator for preaccelerating one bunch at a time atpredetermined time intervals said high-frequencies f, and f beingsubmultiples of the operating frequency f, of said accelerator, and saidfrequencies f, and f being harmonically related.

2. An arrangement as claimed in claim 1, wherein said first and secondcavity resonators are adjacent and high frequency voltage Vcorresponding to said energy at high frequency f applied to said firstcavity resona tor is at least equal to the cut-off voltage V of saidaccelerator.

6. An arrangement as claimed in claim 1, wherein said particles areelectrons.

7. An arrangement as claimed in claim 1, wherein said particles areions.

8. A particle accelerator comprising a preaccelerating arrangement asclaimed in claim 1.

1. A buncher and preaccelerator arrangement for preaccelerating, to apredetermined velocity, particle bunches intended to be injected intothe accelerating sections of a particle accelerator, said arrangementcomprising : a particle source ; means for extracting said particlesfrom said source ; a first cavity resonator and at least one secondcavity resonator positioned along the path of said particles, saidcavity resonators being provided with respective openings for thepropagation of said particles through said cavity resonators ; means forapplying an electromagnetic energy at the highfrequency f1 to said firstcavity resonator for bunching said particles therein ; and means forapplying an electromagnetic energy at the high-frequency f2 to saidsecond cavity resonator for preaccelerating one bunch at a time atpredetermined time intervals ; said high-frequencies f1 and f2 beingsubmultiples of the operating frequency fo of said accelerator, and saidfrequencies f1 and f2 being harmonically related.
 2. An arrangement asclaimed in claim 1, wherein said first and second cavity resonators areadjacent and respectively tuned to frequencies f1 and f2, one of saidsubmultiple frequencies f1 and f2 being an odd multiple of the other. 3.An arrangement as claimed in claim 2, wherein said submultiplefrequencies f1 and f2 are respectively one-half and one-tenth of saidfrequency fo.
 4. An arrangement as claimed in claim 1, wherein saidmeans for extracting said particles are a D.C. voLtage supply.
 5. Anarrangement as claimed in claim 4, wherein said second cavity resonatoris so dimensioned that the sum of the peak values of said supply voltageand of the high frequency voltage V1 corresponding to said energy athigh frequency f1 applied to said first cavity resonator is at leastequal to the cut-off voltage VC of said accelerator.
 6. An arrangementas claimed in claim 1, wherein said particles are electrons.
 7. Anarrangement as claimed in claim 1, wherein said particles are ions.
 8. Aparticle accelerator comprising a preaccelerating arrangement as claimedin claim 1.